1. Field
The invention is directed in general to vehicle brake pads, and more specifically, to brake pad wear sensors to measure the actual amount of wear of a brake pad.
2. Description of Related Art
Vehicle maintenance requires periodic inspection of brake pads in order to assess the deterioration of the brake pads and ultimately determine when the brake pads need to be replaced. Failure to periodically perform this inspection can result in accidents due to brake malfunctions, which can result in danger to vehicle occupants and costly repairs to the vehicle.
Accordingly, brake inspections are required for various types of braking systems, such as drum-based brakes, in which a padded “shoe” presses against the wall of the wheel's drum, and disk brakes, in which two pads press against opposing sides of the rotating wheel disk (rotor). In the disk brake system, the rotor is a hard metal rotating disk, which is attached to the rotating wheel. Two brake pads made of relatively soft material are located on both sides of the rotor at close proximity. When the brake pedal is depressed, the two pads are forced towards the rotor to provide friction force that slows the wheel's rotation. As time progresses, the brake pads can become thinner and may need to be replaced. In most vehicles, it is not obvious to the vehicle operator as to when the brake pads need to be replaced because each wheel often has to be removed to inspect the brake pads. Consequently, the brake pad can totally wear out and the hard metal pad carriers (also called pad holders) can come in contact with the rotor causing substantial damage and creating dangerous and inefficient braking of the vehicle.
In order to improve the inspection process, some vehicles can include embedded sensors to indicate excessive pad wear. As shown in FIG. 1, some systems can include a contact sensor attached in parallel to the internal wall of the pad holder or embedded in the pad at a specific distance from the pad holder. When the pad is worn to a point where it needs to be replaced, the sensor can establish electrical contact with the rotor. An electrical wire can convey the information to the vehicle's dashboard to generate a visual alarm indication for the vehicle operator. As further shown in FIG. 1, some vehicles may employ a distance sensor, which can continuously measure the distance between the two pad holders and thereby provide the information via electrical wires to the vehicle's dashboard.
There are several disadvantages of these systems, namely, undue false positives indicating to the vehicle operator to replace the brake pads. Car manufacturers may incur substantial liability for any false negative occurrences. In other words, if the brake pad is in need of replacement and the system does not generate an alarm, and a consequent accident occurs, the manufacturer may be responsible due to the fact that no alarm was generated. Consequently, the sensors and associated alarm indications may be designed to be overly sensitive to avoid the false negative scenario. Even still, a rupture in the wires connecting the sensor to the dashboard can also produce an alarm not related to the actual condition of the brake pad. This false positive indication can become so frequent and so disturbing to vehicle operators, that many operators may neutralize or deactivate the system entirely, thereby defeating the whole purpose of the system.
Another disadvantage to the “dashboard approach” is that it does not typically provide specific information about which brake pad needs to be replaced. Because brake pads may wear at different rates, the operator will still have to remove all the wheels to determine which pad is in need of maintenance. For vehicles, such as large trucks with many wheels, in which removal of wheels is costly and labor intensive, the dashboard approach can provide little value. Thus, there is a need for devices and methods to allow reliable inspection of brake pads without removing the wheels.